Hyperparathyroidism in Dogs
Veterinary & Aquatic Services Department, Drs. Foster & Smith

Parathyroid gland
Hyperparathyroidism is a condition in which the parathyroid glands (not the thyroid glands!) produce too much parathyroid hormone. Parathyroid hormone is responsible for maintaining the correct balance of calcium and phosphorous in the blood. If blood calcium is low, the parathyroid hormone level increases, which causes calcium to be taken out of the bone to maintain the correct calcium level in the blood and other tissues.

The parathyroid glands are located adjacent to or actually inside of the thyroid glands in the neck. There are two types of hyperparathyroidism.

Primary hyperparathyroidism

Primary hyperparathyroidism occurs when one or more of the parathyroid glands becomes cancerous or abnormally produces excess parathyroid hormone. This causes the calcium levels in the blood to be higher than normal. Primary hyperparathyroidism is generally seen in older animals and is more common in dogs than cats.

Animals with primary hyperparathyroidism may lose their appetite, vomit, drink and urinate more, and may appear drowsy or listless. The signs often come on gradually and considerable organ damage may occur before the animal shows significant symptoms.

The treatment of primary hyperparathyroidism is removal of the abnormal or cancerous gland. The removal of the gland causes a sudden decrease in the amount of parathyroid hormone, and there is a corresponding sudden drop in the blood calcium level. For this reason, the animal must be monitored closely after surgery and given calcium if the level becomes too low.

Secondary (nutritional) hyperparathyroidism

Secondary nutritional hyperparathyroidism is more common and occurs most frequently in puppies and kittens fed an all-meat or organ diet (such as all liver) or a diet with an imbalance of calcium and phosphorous. Meat contains an excess of phosphorous and inadequate amounts of calcium. Although dogs and cats are carnivores, they not only eat meat, but the bones as well, which supply the needed calcium and help to create a better calcium and phosphorous balance.

In secondary hyperparathyroidism, because the calcium intake is low, the parathyroid gland produces more parathyroid hormone, and calcium is removed from the bones.

Puppies and kittens with secondary hyperparathyroidism are often reluctant to move, may stand splay-legged, and may easily develop fractures because of the thinning of the bones. The changes in the density of the bones often cause abnormal growth, especially in the spinal column and pelvis. This places stress on the joints and can result in arthritis.

Treatment basically involves placing the animal on a balanced diet. The skeletal deformities are generally permanent and cannot be corrected.

Secondary (renal) hyperparathyroidism

In dogs with kidney (renal) disease, hyperparathyroidism is a common complication. As the kidneys fail, they are unable to rid the body of phosphorus and also do not produce calcitriol, a hormone that works with parathyroid hormone to maintain the balance of calcium in the body. As a result excess amounts of parathyroid hormone are produced, yet calcium levels remain low. These high levels of parathyroid hormone can result in further damage to the kidneys, as well as the bones and the brain. Treatment of secondary renal hyperparathyroidism include mechanisms to decrease the amoiunt of phosphorus in the blood. This can be accomplished by changing the diet to have lower phosphorus levels and giving phosphate binders, which help to decrease the absorption of phosphorus from the intestinal tract. In addition calcitriol may be given to help increase the calcium levels.

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Reprinted from PetEducation.com.